1. Field of the Invention
This invention relates to wireless communications.
2. Background Information
A network for wireless cellular telephony typically includes a set of fixed base stations that communicate wirelessly with mobile units. In some cases, the individual base stations may be geographically separated from one another. In other cases, more than one base station may be present at a single location. For example, a single site may have three different transmitters, each covering a different 120-degree-wide sector of a horizontal plane. In another example, a single site has six different transmitters, each covering a different 60-degree-wide sector of a horizontal plane. Collocated base stations may also differ in terms of characteristics other than directional orientation (e.g., transmission frequency).
While a mobile unit is typically embodied in a cellular telephone, other devices may also be equipped with mobile units that support voice and/or non-voice data transfer with the network. Such devices include personal digital assistants (PDAs), laptop or notebook computers having integrated or accessory wireless modems (e.g. embodied in PCMCIA cards), and other consumer electronics devices. Mobile units may also be embodied in devices that are intended to operate in generally fixed locations, such as wireless local loop (WLL) units, point-of-sale (POS) units, or other home or business appliances.
During operation, a mobile unit is typically in one of two modes. The mobile unit operates in idle mode when it is not engaged in a communications session (e.g. before a call is initiated or received, or after a call is completed). While in idle mode, the mobile unit listens to a selected channel (e.g. a particular frequency band, code channel, and/or time slot) for a paging message that indicates an incoming call. During idle mode, the mobile unit may also receive control information from (or transmit such information to) the network. After a paging message directed to the mobile unit is received, or when the user initiates a telephone call or other communications session (e.g. a request for data transfer access to support an activity such as e-mail and/or Internet access), the mobile unit enters a dedicated mode.
For portable electronic devices (such as mobile units), it is generally desirable to increase the period of time over which the device may be used before replacement or charging of the power source is needed. Among the subsystems of a mobile unit, the radio-frequency (RF) subsystem is typically one of the greatest power consumers. Therefore, it is generally desirable to limit the amount of time during which the RF subsystem is powered.
One technique currently used to reduce RF subsystem on-time during idle mode is discontinuous reception. For example, a system using this technique may be configured such that a paging message directed to a particular mobile unit will be transmitted by the network only within certain predetermined time intervals. Therefore, the RF subsystem of a mobile unit in idle mode only needs to be powered up and ready to receive during those predetermined time intervals and can be powered down the rest of the time.
From time to time, it may be desirable for the mobile unit to listen for paging messages on a different channel. For example, the propagation path of the channel being monitored by the mobile unit may be subject to change (e.g. as the mobile unit moves from one location to another relative to the base station transmitting on the channel), such that reception of the selected channel could become unreliable. In order for the mobile unit and/or the network to determine whether a different base station—and which other base station—should transmit paging messages that are directed to the mobile unit, a mobile unit typically monitors the quality of channels associated with other base stations. Such monitoring is typically performed during both idle mode (e.g. to support a continued ability to receive paging messages and other broadcast information) and dedicated mode (e.g. to support the ability to continue the communications session between the mobile unit and the network).
The task of monitoring the quality of other channels may give rise to problems. In idle mode, for example, this task may increase the proportion of time during which the RF subsystem remains powered, thus reducing the operating time between battery charges. In dedicated mode, time spent monitoring the quality of other channels may conflict with time needed to perform other on-line tasks such as communication of voice and/or non-voice data.